1. Field of the Invention
This invention relates to a charging member, and a process cartridge and an electrophotographic apparatus which have the charging member.
2. Related Background Art
Image forming apparatus that employ electrophotography, what is called electrophotographic apparatus, are commonly those having an electrophotographic photosensitive member, a charging means, an exposure means, a developing means and a transfer means.
As this charging means, mainly employed is one having a system in which a voltage (a DC voltage only or a voltage formed by superimposing an AC voltage on a DC voltage) is applied to a charging member disposed in contact with, or in proximity to, the surface of an electrophotographic photosensitive member, to charge the surface of the electrophotographic photosensitive member electrostatically.
In the case when the voltage formed by superimposing an AC voltage on a DC voltage is employed as the voltage to be applied to the charging member, an AC power source is necessary which makes the electrophotographic apparatus large or brings about a rise in cost, which may result in a larger power consumption, and which may produce ozone at a high level because of the use of an alternating current that causes a lowering of durability (running performance) of the charging member or the electrophotographic photosensitive member. Accordingly, from this viewpoint, it is preferable that the voltage to be applied to the charging member is only a DC voltage.
In recent years, there has been demand for an electrophotographic apparatus that achieves much higher image quality. Where any conventional charging members are used, however, white or black lines or dots may occur, density non-uniformity due to adhesion of foreign matter or adhesion non-uniformity of foreign matter to the surface of the charging member may occur, or lines or dots due to the profile of the surface of the charging member may occur, depending on certain specific conditions (such as the applied voltage, the environment of image reproduction, and patterns of reproduced images). Such problems have tended to come about especially when the voltage applied to the charging member only a voltage DC voltage.
In addition, in recent years, there has been a demand for an electrophotographic apparatus to achieve much higher running performance, and, as a matter of course, for the charging member to be set therein as well, there has been a demand to achieve much higher running performance.
For example, Japanese Patent Applications Laid-open No. H07-199593 and No. 2000-214657 disclose a technique in which the profile of the surface of the charging member is controlled to improve charging uniformity and running performance or to achieve a reduction in image defects.
At present, there is a demand for an electrophotographic apparatus of much higher added value on the condition that high image quality and high running performance are achieved. An example of such added value may include an apparatus to form good images on transfer materials of various types.
For example, where, e.g., presentation is performed, an overhead projection transparent PET film (hereinafter “OHT”) is used and images are formed thereon in many cases. Also, opportunities are increasing in which electronic image data obtained by photographing with digital cameras or the like are outputted using a printer or photographs are copied using a copying machine. When photographic images are reproduced, specialty paper such as surface-treated paper and high-gloss paper are often used as transfer materials.
Thick and small specialty media such as postcards are also highly frequently used.
The OHT and specialty media often differ in thickness, size, and material compared with plain paper. In order to form good images on these media, the process speed (PS) is often made lower than the case when the plain paper is used as a transfer material, to adapt to such differences.
In order to adapt to transfer materials of various types, which differ in thickness or size and also in material, it is preferable that one electrophotographic apparatus can be set to a plurality of different process speeds. Stated specifically, it means that process speeds can be set to, e.g., ½ speed, ⅓ speed and ¼ speed on the basis of standard speed. Stated more specifically, it means that the apparatus is used at a process speed of 94 mm/second (standard speed) when the plain paper is used as the transfer material, and is used at a process speed switched to 31 mm/second (⅓ speed) when the OHT is used as the transfer material.
However, studies made by the present inventors have revealed that such differences in process speed have a great influence on charging uniformity, and furthermore on the uniformity of reproduced images (image uniformity).
In order to achieve a uniform state in the charging of the electrophotographic photosensitive member, the quantity of electric charges per unit area on the surface of the electrophotographic photosensitive member must be constant. Here, a large quantity of electric charges must be fed to the surface of the electrophotographic photosensitive member per unit time when the process speed is high, and a small quantity of electric charges may be fed when the process speed is low. That is, if the process speed of an electrophotographic apparatus to be used is determined, the charging member may be contrived in conformity therewith.
However, where the electrophotographic apparatus to be used is the electrophotographic apparatus which can be set to a plurality of different process speeds, even if high charging uniformity and furthermore high image uniformity can be achieved at a certain process speed, the charging member becomes over-discharged when the process speed is switched to a speed lower than that, so that a locally over-charged condition may be produced on the surface of the electrophotographic photosensitive member to cause white horizontal lines on reproduced images. Also, when the process speed is switched to a speed higher than that, the charging member becomes under-discharged, so that a locally under-charged condition may be produced on the surface of the electrophotographic photosensitive member to cause black horizontal lines on reproduced images.
This problem tends to occur especially when images are reproduced in a low-humidity environment. This problem also tends to occur especially when the voltage applied to the charging member is only a voltage (i.e., when the AC voltage, having a leveling effect, is not used as the voltage applied to the charging member).
In addition, in the case of the electrophotographic apparatus which can be set to a plurality of different process speeds, the state of static/dynamic contact between the electrophotographic photosensitive member and the charging member, the torque, the state of rubbing, the state of application of voltage to the charging member, and so forth come to change, and hence various stresses tend to be applied to the charging member, compared with an electrophotographic apparatus whose process speed is set to a single value. Hence, the surface of the charging member tends to suffer increased wear, so that the deterioration of the charging member may to make it difficult to maintain high image quality.
This problem also tends to occur especially when images are reproduced in a low-humidity environment or when the voltage applied to the charging member is only a DC voltage.
It has also been found that these problems occur also on the charging member disclosed in Japanese Patent Applications Laid-open No. H07-199593 and No. 2000-214657.
Now, making image quality higher can be achieved to a certain extent by improving charging uniformity. However, as disclosed in Japanese Patent Application Laid-open No. 2000-214657, it has come to light that some image defects, such as black dots, or white dots occurring on reproduced images are caused by the charging member. Such image defects are caused by hills or valleys of the surface of the charging member.
Japanese Patent Application Laid-open No. 2000-214657 discloses a charging member characterized by having no protuberances of 5 μm in height at the surface of the charging member. This charging member enables prevention of the black dots or white dots on reproduced images, but is unable to achieve the charging uniformity adapted to a plurality of different process speeds.
It has also come to light that, in the case when the voltage applied to the charging member is only a DC voltage, the charging uniformity may well come into question, but image defects caused by any defects of the surface of the charging member may become very conspicuous.